A rotary electric machine is a general term for an electric motor, an electric generator, and an electric motor and generator. When an interior magnet rotary electric machine of the present invention is utilized as an electric motor, such an electric motor is called an IP (Interior Permanent Magnet) motor. On the other hand, an electric motor including a rotor in which a permanent magnet is attached to a surface of a rotor core is called an SPM (Surface Permanent Magnet) motor. Furthermore, the IPM motor and the SPM motor are collectively called PM motors.
An ultimate goal of the present invention is to achieve higher-speed rotation of the IPM motor. Patent Literature (PTL) 1, for example, discloses an invention aiming to increase a motor speed, although the disclosed invention relates to the SPM motor instead of the IPM motor.
A rotor of the SPM motor disclosed in PTL 1 includes permanent magnets that are disposed on a surface of a cylindrical yoke, and fiber-reinforced metal wires formed by covering outer peripheral surfaces of high-melting point fibers, which are wound around outer peripheral surfaces of the permanent magnets, with low-melting-point metals, wherein low-melting-point covering portions over the adjacent fiber-reinforced metal wires are fixed to each other through fusion bonding. With the winding of the fiber-reinforced metal wires around the outer peripheral surface of the permanent magnets and the fusion bonding between the low-melting-point covering portions, the strength of the rotor is increased to, be able to resist a large centrifugal force that is applied during high-speed rotation.
An invention disclosed in PTL 2 is intended to increase the strength of electromagnetic steel plates, which are used to constitute a rotor core, in response to a demand for a higher speed of a motor or the like.
In order to rotate the PM motor, such as the IPM motor or the SPM motor, at a high speed, the motor is driven at a high frequency. On that occasion, an eddy current loss generated in a rotor (rotating armature) increases, and demagnetization of a magnet becomes a problem. In general, a stator (stationary armature) is relatively easy to dissipate heat through contact with a casing, a cooling path, etc. On the other hand, the rotor positioned on the inner side of the stator has a difficulty in securing a heat dissipation path for the reason that the rotor is connected to a load via a shaft and that a main heat dissipation path is given as a path extending from the rotor to the casing via the shaft and a bearing. In ordinary motors, because a driving frequency is as low as 50 Hz or 60 Hz, a problem does not particularly arise in relation to the eddy current loss generated in a rotor and to securement of the heat dissipation path. However, when a motor is rotated at a high speed with a frequency of 400 Hz, for example, the eddy current loss generated in a rotor increases. Accordingly, how to reduce the eddy current loss and to secure the heat dissipation path for the rotor are problems to be solved.
In the IPM motor, a magnetic path is formed in an outer peripheral iron core of the rotor core. Because such a magnetic path does not contribute to generation of torque, the rotor core is usually designed in a manner of, for example, thinning an outer peripheral portion of the rotor core as far as possible, and forming a bridge portion through magnetic saturation, or providing a flux barrier. However, when the motor is rotated at a high speed, a centrifugal force increases and hence the outer peripheral portion of the rotor core needs to be thickened. This results in deficiency of torque. In order to compensate for the torque, it is required to increase a current or the number of magnets. Those solutions are problematic in that the former leads to an increase of the eddy current loss and the latter leads to an increase of the cost.
Recently, there are increasing needs for an improvement of energy saving performance and higher efficiency of motors. The IPM motor has received attention as a motor capable of realizing higher efficiency for the reason that, by designing a rotor structure with a salient pole, the IPM motor can utilize reluctance torque in addition to ordinary magnet torque. The need for higher efficiency is also high in motors rotating at high speeds.
The invention disclosed in PTL 1 relates to the rotor of the SPM motor. The SPM motor cannot utilize reluctance torque, and it is harder to achieve high efficiency in comparison with the IPM motor. Furthermore, in PTL 1, the strength of the rotor is increased by fixing the low-melting-point covering portions over the adjacent fiber reinforced metal wires to each other through fusion bonding, and forming a protective layer of the rotor. However, the protective layer is mainly made of a metal material, and a current is easy to flow through the metal material. Thus, although the strength of the rotor can be increased, the eddy current loss cannot be suppressed.
Moreover, in PTL 1, after winding the fiber-reinforced metal wires around the rotor, the fiber-reinforced metal wires are fixed to the rotor through fusion bonding of the low-melting-point metal by heating the fiber-reinforced metal wires with a laser, for example. Accordingly, a step of winding the fiber-reinforced metal wires per rotor is required, man-hours (time) are increased, and a higher cost is needed to fabricate equipment and jigs.
The invention disclosed in PTL 2 is intended to increase the strength of electromagnetic steel plates and is not adapted for coping with the reduction of the eddy current loss.